In order to stimulate and more effectively produce hydrocarbons from oil and gas bearing formations, and especially formations with low porosity and/or low permeability, induced fracturing (called “frac operations”, “hydraulic fracturing”, or simply “fracing”) of the hydrocarbon-bearing formations has been a commonly used technique. In a typical hydraulic fracturing operation, fluid slurries are pumped downhole under high pressure, causing the formations to fracture around the borehole, creating high permeability conduits that promote the flow of the hydrocarbons into the borehole. The high pressure fluids exit the borehole via perforations through the casing and surrounding cement, and cause the oil and gas formations to fracture, usually in thin, generally vertical sheet-like fractures in the deeper formations in which oil and gas are commonly found. The high-pressure fluids typically contain particulate materials called a proppant that is generally composed of sand, resin-coated sand or ceramic particulates. After the proppant has been placed in the fracture and the fluid pressure relaxed, the fracture is prevented from completely closing by the presence of the proppants. As many as 40 separate hydraulic fracturing operations, or stages, can be conducted in a single well.
Tracers have been used in connection with hydraulic fracturing, to provide certain types of diagnostic information about the location and orientation of the fracture(s). This can be accomplished by placing one or more unique tracers in various portions of the fracturing operation, such as in different stages if multiple fracturing stages are performed in the well or in different portions of a stage. Analysis of the produced fluids for the presence of the tracers can provide diagnostic information as to which stages or portions of a stage are in contact with the produced fluids. However, these tracers have been limited to radioactive particles and liquid chemicals of a proprietary nature that are pumped in the fracturing fluid and recovered relatively early in the life of the well.
It would be desirable to have a tracer technology that can provide a very large number of unique tracers that are not subject to degradation at the high temperatures and pressures, along with varying salinity and pH, which are oftentimes found in well formations.